1. Field of the Invention
The present invention relates to a phase change optical recording medium, in which writing, reading and/or erasing of information is conducted by utilizing a change of phase of a recording layer by irradiation with light, and to a process for manufacturing the same.
2. Description of the Related Art
The phase change optical recording medium utilizes, for recording information, a reversible structural change (phase change) between an amorphous state and a crystalline state of a material created by irradiation with light, particularly a laser beam. Such a phase change optical recording medium may have a high storage capacity.
As a result, a phase change optical recording medium with a high speed erase and rewrite of written information performance has been sought. To attain this, it is essential to have a stable repeated overwrite operation. Overwrite means writing new information over previously written information while it is being erased. It is preferred that the possible number of repeatable times of erasing and writing is high.
A phase change optical recording medium is commercially available as a rewritable optical recording disc. For example, a 120 nm phase change optical recording disc is commercially available. A typical disc comprises a stack structure of a polycarbonate, a first dielectric layer of ZnS.cndot.SiO.sub.2, a phase change recording layer of GeSbTe, a second dielectric layer of ZnS.cndot.SiO.sub.2, a reflective layer of an Al alloy and a UV-cured organic resin coated layer in this order.
The level of erase or overwrite repetition durability (cyclability) of these commercially available discs is practically acceptable, but it is still desired that this repetition durability is further improved from the viewpoint of the reliability of the products. It is also desired that the cyclability is further improved from the viewpoint of the production yield since the durability is largely influenced by the conditions of production such as sputtering conditions.
Further, since a high density overwrite disc which is now under development uses a mark edge recording method, deterioration of the recording layer by repeated overwrite significantly adversely affects the quality of the signal and the overwrite repetition durability is important.
It is known that the repeating overwrite characteristics are influenced by various properties of the materials of the recording layer and protecting layers. Thus, as a material for a recording layer, for example, GeSbTe is known. It is reported that a compositional ratio Ge:Sb:Te in a range of 2:2:5 to 2:2.5:5 provides an excellent repetition characteristic. It is also reported that a recording layer in which Cr or N is added to GeSbTe allows further improvement of repetition characteristic.
In a phase change optical recording medium, recording is effected by making a recording layer amorphous by rapid cooling after melting with a high power laser beam irradiation and erasing is effected by making a recording layer crystalline by a gradual cooling after heating with a medium power laser beam irradiation. In order to record and store information, a stable operation in a large number of repetitions of writing and erasing is essential. However, since the write and erase operations involve repetition of rapid heating by more than 300.degree. C., melting and cooling, a recording layer and protecting layers are thermally damaged, the material of a recording layer flows and is deformed, and/or segregation may occur depending on the composition of a recording layer, so that the information quality is deteriorated and the number of stable repeated operation of write and erase is limited.
To improve the write and erase repetition characteristic, there are proposed a selection of a composition of a recording layer with less segregation, a suppression of thermal damage by using a small thermal expansion coefficient material in a protecting layer, provision of an adhesive layer between a recording layer and a protecting layer so as to suppress flow and deformation of a recording layer, and so on.
Also, it is reported that limiting an oxygen content in a recording layer to not more than 2000 ppm can significantly reduce mass flow in the recording layer and allows a stable repetition characteristic be obtained in a wide laser power range (Japanese Unexamined Patent Publication (Kokai) No. 04-329183).
Further, it is reported that inclusion of oxygen in a recording layer improves a repetition characteristic (Japanese Unexamined Patent Publication (Kokai) No. 04-119885). However, the amount of oxygen added to a recording layer is limited, so as not to change the physical properties of the recording layer, by selecting a sputtering atmosphere having an oxygen partial pressure in a range of 5.times.10 Torr to 1.times.10.sup.-4 Torr.
Nevertheless, it is still desired that the rewrite repetition characteristic of a phase change optical recording medium is further improved.
Therefore, an object of the present invention is to solve the above problems, to improve the overwrite and erase repetition durability and to provide a phase change optical recording medium which allows stable rewriting and reading.
The following materials, as examples, have been developed as the materials for a protecting layer: non-oxides such as an Al nitride, a Si nitride, MgF.sub.2 and AlF.sub.3, oxides such as SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2 and TiO.sub.2, chalcogenides such as ZnS, mixtures of a chalcogenide and an oxide such as a mixture of ZnS.cndot.SiO.sub.2, and the like. These materials are deposited as a thin layer by a known method and are used as protecting layers but the obtained discs do not allow a sufficient number of repetitions of overwriting.
The protecting layer of a phase change optical recording medium must be excellent in heat resistance and mechanical properties, since it is subjected to thermal and mechanical loads during writing and erasing, and also must have a function of protecting the recording layer during storage of the optical medium. Further, it must have a low thermal conductivity from the viewpoints of recording sensitivity and repetition durability.
The protecting layer of an oxide or nitride has a weak adhesive force with a recording layer comprising a chalcogen. As a result, during storage in a high temperature and humidity environment, the protecting layer may peel off or cracks may be formed in the protecting layer. Partly due to a weakness in the adhesive force between the protecting layer and the recording layer, heating supplied to the recording layer from a laser beam is not dissipated at an appropriate rate and the recording marks are connected with each other and the material may flow in one direction, resulting in reduction in the number of erase and overwrite repetitions.
Some methods have been proposed to improve the erase and overwrite repetition property. For example, Japanese Unexamined Patent Publication (Kokai) No. 06-139615 reported that provision of an adhesive layer of Al.sub.2 O.sub.3, GeO.sub.2, SiO.sub.2, Ta.sub.2 O.sub.5, Y.sub.2 O.sub.3 or the like between a protecting layer and a reflective layer and/or between a protecting layer and a recording layer improves the number of erase and overwrite repetitions.
Japanese Unexamined Patent Publication (Kokai) No. 06-314439 reported that in an optical recording medium comprising a substrate, a first dielectric layer of ZnS.cndot.SiO.sub.2, a second dielectric layer of ZnS.cndot.SiO.sub.2, a recording layer, a third dielectric layer of ZnS.cndot.SiO.sub.2, a fourth dielectric layer of ZnS.cndot.SiO.sub.2, and a reflective layer, the layer strength is improved by including nitrogen in the first and/or fourth dielectric layers, and adhesion of a recording layer and protecting layers is improved by deposited the second and third dielectric layers in a rare gas atmosphere.
The effect of improving the number of erase and overwrite repetition is not sufficient in each of the above proposals.
In accordance with an investigation by the inventors, the following conditions must be met to have a sufficient erase and overwrite repetition durability: (1) The wettability between an adhesive layer and a recording layer of a chalcogenide which is fused and made amorphous must be excellent, and the adhesive layer must not deteriorate by being repeatedly exposed to high temperatures. (2) The layers must have a sufficient adhesion with each other. (3) The adhesion of the adhesive layer with the reflective layer must be excellent.
In the prior art, only the adhesion of the adhesive layer with the recording layer is considered but the adhesion of the adhesive layer with the reflective layer is often not considered, which is one of the reasons why a sufficient erase and overwrite repetition durability was not obtained.
Since an excess laser power may deteriorate the repetition durability, the heat conduction coefficients of the adhesive layer and the protecting layer must be sufficiently small, in order to have a sufficient recording sensitivity at a low laser power, which is required for a practical disc medium.
From these requirements, the above reports and proposals of the prior art may be considered as below:
Japanese Unexamined Patent Publication (Kokai) No. 06-139615 adopts an adhesive layer of an oxide. It is considered that a chalcogen-containing recording layer has a low affinity with an oxide and therefore does not have a sufficient adhesion with the oxide adhesive layer. Even if adhesion between an oxide adhesive layer and a non-fused recording layer is excellent, the wettability between an oxide adhesive layer and a recording layer is low, which results in repellence and flow of the fused material of the recording layer and results in insufficient repetition durability of the disc. It is also supposed that one of the reasons for the insufficient repetition durability is a lack of consideration of the heat resistance of the protecting layer, which causes a change in the crystallinity of the protecting layer by repeated heating and thus deterioration of the repetition durability.
In Japanese Unexamined Patent Publication (Kokai) No. 06-314439, a dielectric layer of ZnS.cndot.SiO.sub.2 deposited in a rare gas atmosphere may tend to have oxygen in an amount less than the stoichiometric amount and therefore have a non-smooth or rough surface, and a recording layer formed on the non-smooth surface of the dielectric layer has a weak bond to the dielectric layer and may easily be peeled or cracked during repeated overwriting and erasing. This is considered to be one reason for a insufficient repetition durability. Control of the oxygen content requires deposition under delicate conditions or under narrow conditions, which decreases the productivity. Although a dielectric layer of ZnS.cndot.SiO.sub.2 containing nitrogen and/or oxygen has an improved layer strength, the adhesive force of the layer to a substrate and a reflective layer is low so that the repetition durability is lowered.
The object of the present invention is to solve the above problems, improve the adhesion of a protecting layer with a recording layer and provide a phase change optical recording medium without peeling, cracking or flow of a layer.